Problem: $\dfrac{7}{4} - \dfrac{9}{12} = {?}$
Explanation: $ = {\dfrac{7 \times 3}{4 \times 3}} - {\dfrac{9 \times 1}{12 \times 1}} $ $ = {\dfrac{21}{12}} - {\dfrac{9}{12}} $ $ = \dfrac{{21} - {9}}{12} $ $ = \dfrac{12}{12}$